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For insertion of a medial UKA, 5° to 7° of true
varus is acceptable (on corrected views),
corresponding to a 10° deformity on pre-
operative goniometry [7, 10, 12]. For the lateral
compartment, an upper limit of 7° of post-
reduction valgus is the upper limit,
corresponding to 12° of uncorrected valgus
[31, 36]. Some authors have proposed wider
inclusion limits, judging deformity in the
context of the weight of the patient [6]. Gulati
et al.
[10] propose that axial deformity should
not have an upper limit, but should always be
restored, without affecting clinical outcomes or
durability of implants. It is important to note
that reducibility should be quantitative –
showing satisfactory re-alignment, but also
qualitative, as judged by a repositioning of the
tibia in line with the femur.
Tibial slope can be considered under the
category of saggital alignment. Reproducing
the slope is the aim with the UKA, to accurately
restore knee kinematics. Tibial slopes greater
than 10° are associated with poorer results, and
normally considered a contra-indication to
surgery. When weight-bearing the tibial slope
determines anterior tibial translation, and
contributes to constraints on the posterior
aspect of the tibial plateau. Potential
consequences for an increased slope include
early polyethylene wear, fatigue rupture of the
ACL [12] and progressive risk of tibial plateau
collapse [1].
Range of Motion
The presence of an established restriction in
flexion or a limitation in extension frequently
translate to marked OA changes, and contra-
indicate using a unicondylar knee prosthesis.
10°-15° of flexion contracture, and extension
no greater than 100° are considered threshold
values [3, 31]. In cases where the cause of
stiffness is extra-articular, this contra-indication
becomes a relative one. It is important to
counsel such patients that their restriction in
ROM will persist after surgery.
Patient Factors
Age
For most authors the ideal age for considering
a UKA is, like most arthroplasty, between 60-
65 years old [7, 23]. For older patients – over
80 years old – there are concerns regarding the
quality of bone for such an implant, and these
patients are associated with a higher risk of
medial tibial plateau collapse, especially in the
context of established osteoporosis [1]. The
risks of using a UKA must be balanced against
what is a significant benefit for such frail
patients – the reduction in the magnitude of the
surgical insult. The older the patient, the higher
the likelihood that technical faults or ancillary-
related difficulties will cause bony collapse
[34]. For younger patients (those under 65 years
old) a legitimate alternative is a tibial osteotomy.
Despite this, the improved quality of implants
and in particular encouraging clinical outcomes
have led to surgeons reduce the age at which
they would consider a UKA. Certain authors
have demonstrated survival rates equivalent
and superior to osteotomy for sporting activity
and for quality of life [15, 30]. Despite this, the
higher rate of revision in patients under 65 years
old reported by national registries should
temper this enthusiasm [40]. Finally, revision
of UKA to TKR remains a relatively
straightforward option which gives better
results than either TKR post HTO or revision
of TKR to TKR. This makes consideration of
UKA a strong argument for a younger patient
for whom the prospect of a revision procedure
is to be expected regardless of the index
intervention [17, 19]. For younger patients
revision of UKA to UKA, or isolated change of
polyethylene are manageable interventions
assuming that serial observations and follow
up continue and reveal no evidence of aseptic
loosening [30].
Weight and size
Although recent publications have questioned
weight as a limiting factor consensus for the